| callNextMethod {methods} | R Documentation |
A call to callNextMethod can only appear inside a method
definition. It then results in a call to the first inherited method
after the current method, with the arguments to the current method
passed down to the next method. The value of that method call is the
value of callNextMethod.
callNextMethod(...)
... |
Optionally, the arguments to the function in its next call
(but note that the dispatch is as in the detailed description below;
the arguments have no effect on selecting the next method.)
If no arguments are included in the call to callNextMethod, the
effect is to call the method with the current arguments.
See the detailed description for what this really means.
Calling with no arguments is often the natural way to use callNextMethod; see the examples.
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The “next” method (i.e., the first inherited method) is defined
to be that method which would have been called if the current
method did not exist. This is more-or-less literally what happens: The
current method (to be precise, the method with signature given by the
defined slot of the method from which callNextMethod is
called) is deleted from a copy of the methods for the current generic,
and selectMethod is called to find the next method (the
result is cached in a special object, so the search only typically
happens once per session per combination of argument classes).
Note that the preceding definition means that the next method is
defined uniquely when setMethod inserts the method containing
the callNextMethod call, given the definitions of the classes
in the signature. The choice does not depend on the path that gets us
to that method (for example, through inheritance or from another
callNextMethod call). This definition was not enforced in
versions of R prior to 2.3.0, where the method was selected based on
the target signature, and so could vary depending on the actual
arguments.
It is also legal, and often useful, for the method called by
callNextMethod to itself have a call to
callNextMethod. This generally works as you would expect, but
for completeness be aware that it is possible to have ambiguous
inheritance in the S structure, in the sense that the same two
classes can appear as superclasses in the opposite order in
two other class definitions. In this case the effect of a nested
instance of callNextMethod is not well defined. Such
inconsistent class hierarchies are both rare and nearly always the
result of bad design, but they are possible, and currently undetected.
The statement that the method is called with the current arguments is
more precisely as follows. Arguments that were missing in the current
call are still missing (remember that "missing" is a valid
class in a method signature). For a formal argument, say x, that
appears in the original call, there is a corresponding argument in the
next method call equivalent to “x = x”. In effect, this
means that the next method sees the same actual arguments, but
arguments are evaluated only once.
The value returned by the selected method.
The R package methods implements, with a few exceptions, the programming interface for classes and methods in the book Programming with Data (John M. Chambers, Springer, 1998), in particular sections 1.6, 2.7, 2.8, and chapters 7 and 8.
While the programming interface for the methods package follows
the reference, the R software is an original implementation, so
details in the reference that reflect the S4 implementation may appear
differently in R. Also, there are extensions to the programming
interface developed more recently than the reference. For a
discussion of details see ?Methods
and the links from that documentation.
Methods for the general behavior of method dispatch
## some class definitions with simple inheritance
setClass("B0" , representation(b0 = "numeric"))
setClass("B1", representation(b1 = "character"), contains = "B0")
setClass("B2", representation(b2 = "logical"), contains = "B1")
## and a rather silly function to illustrate callNextMethod
f <- function(x) class(x)
setMethod("f", "B0", function(x) c(x@b0^2, callNextMethod()))
setMethod("f", "B1", function(x) c(paste(x@b1,":"), callNextMethod()))
setMethod("f", "B2", function(x) c(x@b2, callNextMethod()))
b1 <- new("B1", b0 = 2, b1 = "Testing")
b2 <- new("B2", b2 = FALSE, b1 = "More testing", b0 = 10)
f(b2)
f(b1)